The seat belt systems used almost universally in automobile and other road vehicles for passenger protection in an emergency, such as a collision or abrupt stop, keep the passsenger from being thrown forward. In some types of emergency, however, the passenger is not only thrown forward but is thrust down on the seat, an occurrence that can be termed a "plunging" or "submarining" effect. In other words, although the seat belt system restrains forward and upward motion of the passenger, it does not restrain downward motion, so if the vehicle rapidly accelerates upwardly, the inertia of the passenger creates the plunging effect; the passenger abruptly moves down relative to the seat, a situation that is readily permitted by the soft, spring and foam-cushioned seats conventionally used in road vehicles. The relative downward movement of the passenger has at least two possible adverse results. First, the seat belt system is, in effect, loosened, thus increasing the degree of relative forward movement as compared to what would be possible without a plunging effect. Second, the shoulder belt rests higher than is desirable and may even catch the passenger's neck, a very dangerous situation.
One way of avoiding or mitigating the plunging effect is to provide a very stiff and hard seat, and vehicle seats have been made in the form of a contoured steel plate covered with a thin foam cushion. Unfortunately, though this type of seat is probably safer than the conventional soft, springy seats, it is very uncomfortable and, therefore, is commercially and practically unacceptable as a replacement for conventional vehicle seats.